Re: Access rules for current->memcg

Nikolay Borisov
2015-07-16T21:21:51Z

On Thu, Jul 16, 2015 at 6:22 PM, Michal Hocko <mhocko-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:
> On Thu 16-07-15 18:11:48, Nikolay Borisov wrote:
>>
>>
>> On 07/16/2015 05:59 PM, Michal Hocko wrote:
>> > On Thu 16-07-15 16:34:08, Nikolay Borisov wrote:
>> >> Hello,
>> >>
>> >> I'd like to ask what are the locking rules when using
>> >> mem_cgroup_from_task(current)? Currently I'm doing this under
>> >> rcu_read_lock which I believe is sufficient. However, I've seen patches
>> >> where reference is obtained via mem_cgroup_from_task and then
>> >> css_tryget_online is used on the resulting cgroup?
>> >
>> > RCU will guarantee that the memcg will not go away. The rest depends on
>> > what you want to do with it. If you want to use it outside of RCU you
>> > have to take a reference. And then it depends what the memcg is used
>> > for - some operations can be done also on the offline memcg.
>> >
>> > Btw. mem_cgroup_from_task is not the proper interface for you. You
>> > really want to do
>> > memcg = get_mem_cgroup_from_mm(current->mm)
>> > [...]
>> > css_put(&memcg)
>>
>> Unfortunately this function is static, do you think there might be any
>> value of a patch that exposes it upstream?
>
> Ohh, you are right! I thought I made it visible with my recent changes
> but nope. There are no external users currently.
>
> Could you tell us more why it would be useful for you?

In my particular use case I have to query the memcg's various counters to expose
them to the user in a different way than via the cgroup files
(memory.limit_in_bytes etc).

> --
> Michal Hocko
> SUSE Labs